Quasi-Z-Source Inverters in Renewable Power Systems Modeling

Abstract

This paper is devoted to in-detail demonstrationand description of the overall functionality and deailed processes in every component of quasi-z-source voltage inverter. Expressions for currents and voltages on all elements of the circuit in standard and "shoot-trough" modes are provided. These ratios demonstrate one of the main technological advantages of the quasi-z-source topology: which is the possibility of the voltage reduction on one of the capacitor that forms the quasi-z-circuitry that led to the overall size and weight reduction of the resulting device and additional reliability increase. Matlab calculations,that are provided in the practical part of the article,fully confirmed the theoretical dependencies. At the same time, obtained practical results of the simulation demonstrated the main advantage of the quasi-impedance topology - the ability to provide a continuous current of the input source without zero pauses. That is additionally confirmed by a graphical representation of transients and the spectrum of the output voltage of the inverter. As the second topic of the article, the problem of the control system operating principle selection was discussed. As it was determined in the process of the previous research and practical modeling of the quasi-z-source inverter, providing a switching of the inverter valves on the frequency that equal to the frequency of the desired output voltage, which for the most of the world’s electrical power systems is equal to 50-60 Hz, is leading to a size and mass increase of the quasi-z-source circuitry, according to its design ratios. Additionally, such component are either not represented in the modern electronic components lineup or providing sufficient energy losses that in some design cases could neglect the voltage boost effect of the quasi-z-source circuitry implementation. To overcome discovered limitations, several PWM control methods were suggested. All of them could be divided into two groups: classic and vector PWM methods. Classic methods are based on combination of the basic PWM modulation techniques that are widely used in conventional full-bridge invertors with the insertion of the “shoot-trough” state activation mode. A model of the control system for the classical approach is offered. It is shown that the method of simple control of the state of "breakdown" is based on the placement of time intervals of the inverter in the state of "breakdown" within the normal period of operation of the bridge inverter. The only difference between the models is the inclusion of an additional constant signal exceeding the amplitude of which the carrier leads to the activation of the “shoot-trough” state.In conclusion the discussion on topic of the classic control methods implementation in up-to-date designs and possibility of its combination with a different modern approaches aimed on a quasi-z-source topology parameters modification are provided.